7.6 Message Oriented Middleware

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The Message Oriented Middleware (MOM) consists of a transport layer that deals with the communication (physical) aspects of moving messages between the origination and the destination points. It takes care of the networking protocol issues, the message encoding and even translation. It is said to encapsulate the business of transporting message. At the same time, it provides APIs to enable application to manipulate (send/receive) messages. It is obviously a very critical and absolutely necessary part of the messaging-based integration layer.

The transport layer, per se, does not do any message transformation or handle message routing.

From the integration layer perspective, such basic messaging systems offer limited value. They were therefore enriched with the additional functionality of handling the transformation and containing dynamic routing capabilities. This in turn evolved even further to support not only point-to-point distribution but also publish/subscribe style scenarios and handling of the complex message flows. The messaging system components providing this functionality are commonly known as the message brokers, for example, WebSphere MQ.

Message broker services

A message broker is a server-side service in a message-oriented system. It manipulates the messages it receives, and performs some or all of the following functions:

Routing services:
- Content based
- Publish/subscribe style
- Message flow processing
Data transformation (mapping message to other formats) services:
- Syntactic (format driven)
- Semantic (content driven)
Security (authentication/authorization, encryption/decryption) services
Transactional service (unit of work support)
Error handling
Service invocation (invoking an external function to process message payload)

A message flow is a sequence of operations on a message, dependent upon the message content and the current message flow state. Message flows can be viewed as business services, initiated by receiving a message.

If the broker supports the publish/subscribe style of messaging, subscribing applications can register their topic subscriptions with the broker. Publishing applications can then send messages to the broker, who will map the topic to the registered subscribers and forward the message to all interested parties.

A very good example of the messaging system providing an extensive set of the message broker services is the WebSphere Business Integration Message Broker product.

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